1. Field of the Invention
The present invention relates to a substrate for a magnetic recording medium and a magnetic recording medium which comprises a recording layer.
2. Description of the Related Art
In the field of magnetic recording, hard disk devices are indispensable as primary external recording devices of computers, for example personal computers, for recording information. Accompanying an increase in hard disk drive recording densities, development of perpendicular types of magnetic recording which allow a higher recording density is progressing, replacing conventional longitudinal magnetic recording methods.
In perpendicular magnetic recording, a magnetic field from an adjacent bit is in the same direction as the magnetizing direction, forming a closed magnetic circuit between the adjacent bits, and self-reducing magnetic field (referred to below as a “demagnetizing field”) caused by its own magnetic field is less than in horizontal magnetic recording, stabilizing the magnetizing condition.
In perpendicular magnetic recording there is no particular necessity to make the magnetic film thin with increases in recording density. From these points, because perpendicular magnetic recording can reduce the demagnetizing field and secure the KuV values wherein Ku represents anisotropic energy, and particularly crystalline magnetic anisotropic energy in the case of magnetic recording, and V represents a unit recording bit volume, it has stability against magnetization by thermal fluctuations, and it can be said that this is a recording method that makes it possible to push the recording limit significantly upward. As recording media, perpendicular recording media have a high affinity with horizontal recording media, and it is possible to use basically the same technology as was used conventionally in both reading and writing of magnetic recording.
As for perpendicular magnetic recording media, there has been extensive investigation into double layered perpendicular magnetic recording media in which, in addition to a recording layer, a soft magnetic lining layer (typically permalloy or the like), a recording layer (for which candidate materials include CoCr-based alloy, a multi-layer film of alternating laminated layers of ultra thin films of PtCo layer and Pd and Co, and SmCo amorphous film), a protective layer and a lubricating layer are layered in this order on a substrate.
The double layered perpendicular magnetic recording medium has much better writing properties than a perpendicular magnetic recording medium having only a recording layer as a magnetic functional layer.
It is necessary that the lining layer of the double layered perpendicular magnetic recording medium is soft magnetic, and has a film thickness of at least 100 nm to about 500 nm. The soft magnetic lining layer is the conductive path not only for magnetic flux from the recording film above it, but also for the writing flux from the recording head. Because of this, it plays the same roll as an iron yoke in the magnetic circuit of a permanent magnet so that it has to be much thicker than the recording layer.
Compared to the formation of non-magnetic Cr-based primer film in a horizontal recording medium, the formation of the soft magnetic lining film of the double layered perpendicular recording medium is not simple. Ordinarily, the films constituting a horizontal recording medium are all formed by a dry process (principally by magnetron sputtering) (see Japanese Patent Application Unexamined Publication No. 5-143972/1993). Methods for forming not only the recording layer but also a soft magnetic layer by dry processing have been investigated for the double layered perpendicular recording medium as well. However, from the aspect of mass-production and productivity, there are large problems with forming soft magnetic layers by dry processing because of process stability, the complexity of parameter settings, and more than anything else, process speed. Furthermore, for the purpose of achieving higher densities, it is necessary to make the height at which the head floats above the surface of the magnetic disk, the flying height, as low as possible and in the manufacture of the double layered perpendicular magnetic recording medium, it is necessary to cover the substrate with a metal film of such a thickness that it can be leveled by grinding. However, because the adhesion of a thick film obtained by a dry process is low, leveling by grinding is very problematic. Thus, various tests were performed to cover a non-magnetic substrate with a metal film by a plating method, with which a thick film can be formed more easily than by vacuum deposition.